1. Field of the Invention
The present invention relates to a laser irradiation equipment that crystallize a semiconductor film using laser light and activate after an ion implantation.
2. Description of the Related Art
In recent years, a technique of forming a TFT on a substrate has greatly progressed, and its application and development for active matrix semiconductor display device has been advanced. In particular, since a TFT using a polycrystalline semiconductor film has higher field-effect mobility than a TFT using a conventional amorphous silicon film, it enables high speed operation. Therefore, although the pixel is conventionally controlled on a driving circuit provided outside the substrate, it is possible to control the pixel on the driving circuit formed on the same substrate.
Incidentally, as the substrate used in the semiconductor device, a glass substrate is expected hopefully as the substrate in comparison with a single crystal silicon substrate in terms of the cost. Since a glass substrate is inferior in heat resistance and is susceptible to heat-deformation, in the case where a polysilicon TFT is formed on the glass substrate, laser annealing is used for crystallization of the semiconductor film in order to avoid heat-deformation of the glass substrate.
Characteristics of laser annealing are as follows: it can greatly reduce a processing time in comparison with an annealing method using radiation heating or conductive heating; and it hardly causes thermal damage to the substrate by selectively and locally heating a semiconductor or the semiconductor film.
Note that the laser annealing method here indicates a technique of recrystallizing the damaged layer formed on the semiconductor substrate or the semiconductor film, and a technique of crystallizing the amorphous semiconductor film formed on the substrate. Also, the laser annealing method here includes a technique applied to leveling or surface reforming of the semiconductor substrate or the semiconductor film. A laser oscillation apparatus applied is a gas laser oscillation apparatus represented by an excimer laser or a solid laser oscillation apparatus represented by a YAG laser. It is known as the apparatus which performs crystallization by heating a surface layer of the semiconductor by irradiation of the laser beam in an extremely short period of time of about several ten nanoseconds to several hundred microseconds.
Lasers are roughly divided into two types: pulse oscillation and continuous oscillation, according to an oscillation method. In the pulse oscillation laser, an output energy is relatively high, so that mass productivity can be increased assuming the size of a beam spot to be several cm2 or more. In particular, when the shape of the beam spot is processed using an optical system and made to be a linear shape of 10 cm or more in length, it is possible to efficiently perform irradiation of the laser beam to the substrate and further enhance the mass productivity. Therefore, for crystallization of the semiconductor film, the use of a pulse oscillation laser is becoming mainstream.
However, in recent years, in crystallization of the semiconductor film, it is found that grain size of the crystal formed in the semiconductor film is larger in the case where the continuous oscillation laser is used than the case where the pulse oscillation laser is used. When the crystal grain size in the semiconductor film becomes large, the mobility of the TFT formed using the semiconductor film becomes high and variation of the TFT characteristics due to a grain boundary is suppressed. Therefore, a continuous oscillation laser is recently attracting attention.
However, since the maximum output energy of the continuous oscillation laser is generally small in comparison with that of the pulse oscillation laser, the size of the beam spot is small, which is about 10−3 mm2. Accordingly, in order to treat one large substrate, it is necessary to move a beam irradiation position on the substrate upward and downward, and right and left, it results in increasing the processing time per one substrate. Thus, processing efficiency is poor and it is an important object to improve the processing speed of the substrate.
A technique for high processing efficiency of substrate by overlapping and condensing a plurality of beam spots to form one beam spot is well known (For example, Patent Literatures 1 and 2).    Patent Literature 1: Japanese Publication of Laid-Open Patent Application No. Hei. 5-315278 (FIG. 11)    Patent Literature 2: Japanese Publication of Laid-Open Patent Application No. Hei. 4-282869 (second to third pages, FIG. 1A)